The invention discloses a purification technology for a side product of flue gas ammonia-desulphurization, by the purification technology, the problem of influence on product quality caused by that the dust cannot be removed in the existing purification technology is solved. The technical scheme comprises the following steps of: 1, solid liquid separation: heating an absorption solution to be 75 DEG C to 90 DEG C, standing, settling and separating, so as to obtain an oil phase of the uppermost layer, a water phase of the intermediate layer, and a solid-liquid mixed phase of the lower layer; 2, film filter oxidization: introducing the water phase in a film filter oxidization device, bubbling while filtering, introducing hot air, keeping the hot air at the temperature of 75 DEG C to 90 DEG C, carrying out oxidization of ammonium sulfate and ammonium nitrite, and film filter treatment of dust, so as to obtain a gas phase, a filtrate and a solid-liquid mixed phase; 3, concentrating cooling: introducing bubbles in the filtrate in room-temperature air, or spraying the filtrate to contact room-temperature air, concentrating and cooling to 55 DEG C to 60 DEG C, so as to obtain a gas phase and a cooled solution, and introducing the cooled solution in an ammonium sulfate crystallizing system to carry out the ammonium sulfate crystallization treatment. The technology disclosed by the invention is simple in process, low in operation cost, is doubled as oxidization and dust removing, and high-quality ammonium sulfate products can be obtained.